The need for precise alignment of magnetic tape as it is wound onto a roll is well known. U.S. Pat. No. 4,909,455 describes the need for winding magnetic tape in such a way that each succeeding wrap, or strand, of tape is wound onto the roll of tape in a position which is laterally precisely superimposed over the tape already wound onto the roll. Lateral movement or placement of the tape is across the width of the tape, in a direction substantially parallel to the winding axis. The process of assuring precise superposition of succeeding strands of tape on the roll is called strand alignment. Failure to achieve adequate strand alignment can result in an unsightly appearance of the sides of the roll, as well as damage to the edges of the tape on the roll.
The problem of achieving precise strand alignment becomes acute when winding large rolls, called pancakes, of magnetic recording tape, because the tape wound onto such rolls is typically 0.5 in wide and of such length as to produce a pancake having an outside diameter of 35-41 cm (14-16 in), when wound on a hub having a diameter of only about 11.4 cm (4.5 in). Since the reels used for holding such pancakes consist merely of a hub, with no side flanges which might improve strand alignment and shield the sides of the roll from view, it is important that something aligns the tape strands during winding.
Strand alignment becomes more difficult as winding speeds increase, due to the tendency of the rapidly moving strand and roll surfaces to drag boundary layers of air into the space between the strand being wound and the existing roll of tape. At very high winding speeds, one or more of the outer strands of tape on the roll can float on a cushion of air entrained between the strands. Even the slightest variation in tape properties or mechanical disturbance to the floating strands can lead to noticeable misalignment of the strands on the roll. Despite the difficulties created by attempting to wind magnetic tape at very high speeds, the need for increased productivity in tape manufacturing demands increasing speeds in all phases of the manufacturing process, including winding.
Several systems for improving strand alignment during high speed winding are known. One system blocks the boundary layers of air on the tape strand and on the roll. U.S. Pat. No. 5,039,023 discloses a device which uses small rollers in contact with the roll and the strand for blocking the boundary layer of moving air. This device appears to be useful in reducing the air cushion formed between strands of tape, but still requires additional elements to achieve the actual alignment of the strands. Also, this device is mechanically complex.
Another system for improving strand alignment provides a reference plane, or flange, coincident with one side of the roll, and urges the strands of tape against this flange during winding, providing positive strand alignment. U.S. Pat. No. 4,789,110 discloses a winding apparatus in which the strand of magnetic tape is subjected to a magnetic field which pulls it toward an alignment flange. To facilitate movement of the tape by the relatively weak magnetic forces, air is blown into the interface region to enhance the lubricating effect of the air cushion between the strands. Air is then removed by providing a vacuum at another location on the roll, which fixes the strands in place. This patent also discloses mechanical belts for urging strands of tape toward the reference flange. U.S. Pat. No. 4,842,210 discloses a pressure disk, which acts in cooperation with a magnet, to press the strands of tape against a reference flange.
Although flanges can achieve strand alignment in magnetic tape winding, they present certain difficulties. In particular, tape reels with flanges are heavier and more expensive than flangeless hubs. Further, because the flanges have a large diameter, while being very thin, they are easily broken. Also, in slitting operations, several rolls of tape are wound close to each other, and space may not be available to accommodate flanged reels.
Another known system for improving strand alignment attaches the strand of tape to the roll immediately upon contact of the strand with the roll. In its simplest form, immediate attachment involves contacting the roll with a pressure roll, or packwheel, at the point of contact of the strand and the roll, to press the strand against the roll. However, using packwheels in high speed winding requires prohibitively high contact pressures to remove the boundary layers of air which are dragged along with the tape and roll when the winding speed is high.
An alternative for achieving immediate strand attachment is disclosed in U.S. Pat. No. 4,909,455, in which the tape is magnetized so that the strand being wound is attracted to the roll and is fixed in place as it is wound. Similarly, the strand can be attached to the roll electrostatically. Japanese Kokai 57-57152 discloses the application of an electrical charge onto an electrically insulating magnetic tape before winding onto a roll. Similarly, 3M's U.S. patent application Ser. No. 07/928,153 discloses an apparatus for improving the winding of tape which is not electrically insulating, but contains antistatic agents; applying electrical charges in combination with winding at a sufficiently high speed fixes the strand to the roll before the electrostatic charge has time to dissipate.
These immediate attachment systems are useful in improving strand alignment, but they only remedy that portion of strand misalignment caused by air entrainment, and provide no positive positioning of the strand as it is wound onto the roll. In addition, these immediate attachment systems involve apparatus which may be difficult to install and use in production, and which may be unfamiliar to production workers.
Still another system for improving strand alignment uses edge control rollers, as disclosed in U.S. Pat. No. 4,778,119. As shown in FIG. 1 of this patent, edge control rollers hold the strand of tape in alignment with the roll. As shown in FIG. 2 of this patent, the edge control rollers include tapered flange surfaces, which align the edges of tape with respect to the roll of tape, and which continuously hold and adjust the strands of tape on the roll. This strand alignment system is purely mechanical, and is familiar to production workers. In addition, it provides positive positioning of the strand on the roll, while holding strands in place until the air cushion can flow out of the space between them.